The present invention relates to semiconductor devices including insulated gate field-effect transistors (IG FET) and a manufacturing method of the same, and more particularly to a high speed switching IG FET. The present invention is effective in an application to a lifetime control technique of a built-in diode of an IG FET by the use of, for example, electron beam irradiation.
In a power MOSFET, the lifetime control is performed by diffusing a heavy metal or radiating an electron beam or a neutron beam in order to reduce reverse recovery time t.sub.rr of the built-in diode. For example, a technique of adjusting reverse recovery time of a built-in diode by applying electron beam irradiation and annealing treatment to a substrate in which a MOS semiconductor element is formed is disclosed in JP-A-62-219664 (laid open on Sep. 26, 1987).
A polyimide resin has less pin holes as compared with a phosphosilicate glass (PSG) film and can be formed thicker. Thus, the polyimide resin is considered to be suitable as a surface passivation film for .alpha.-ray resistant or high withstand breakdown voltage semiconductor elements, and is widely used not only for bipolar transistors, but also for insulated gate field-effect transistors, IC devices and the like. For example, a technique of coating a surface of a semiconductor chip with a polyimide resin for .alpha.-ray shielding is disclosed in JP-A-55-156343 (laid open on Dec. 5, 1980). Further, a new polyimide film for multilayer interconnection and polyimide resins for semiconductors are described in a Japanese journal "DENSHI ZAIRYO" ("Materials for Electronics") Dec. 1, 1988, published by Kogyo Chosakai, pp. 41-45 and pp. 46-52.
In reducing the reverse recovery time t.sub.rr in a built-in diode of a power IG FET (a power MOSFET), the life-time control by electron beam irradiation is performed. That is, with this technique, after a MOS transistor section including p-type wells is formed in an n-type semiconductor substrate, electron beam is radiated onto the transistor section and then annealing treatment is performed, thereby to reduce the reverse recovery time of junction diodes (built-in diodes) between an n-type substrate and a p-type wells, thus making it possible to utilize the built-in diode positively as a flywheel diode.
Further, as shown in FIG. 5 of the above-mentioned JP-A-62-219664, a threshold voltage V.sub.th of MOSFETs irradiated with an electron beam recovers approximately 80% at the maximum of V.sub.th of MOSFETs which are not irradiated with an electron beam. Furthermore, the rate of recovery of these values V.sub.th depends on the length and duration time of annealing treatment, and saturates in a predetermined treatment time at a fixed temperature. Thus, it will be possible to always obtain a maximal recovery rate by selecting the time for the annealing treatment.
On the other hand, a polyimide resin has been used for a passivation film and an inter-layer insulating film due to its superiority in the dielectric breakdown voltage (withstand voltage), alpha rays shielding effect, thick film formation, buffer performance and the like.